This study attempts to define mechanisms of stimulation of gastric secretion and gastrin release, and the relation between the two. Gastric secretion of H plus, Cl minus and pepsin and gastrin release are being studied in conscious gastric fistula dogs with intact vagi, with fundic vagotomy and with truncal vagotomy. Stimuli (histamine and various H-2 analogs, pentagastrin or G-17, urecholine, 2-deoxy-D-glucose (2DG), food and bombesin) are given in graded doses alone or in combination with antagonists (H-2 antagonists, atropine, somatostatin, PGE2). In addition pulse and blood pressure, representing H-2 effects, are being measured. The pathophysiology of gastric secretion and gastrin release are being studied in patients with Zollinger-Ellison syndrome, and duodenal and gastric ulcer, compared with controls. Pentagastrin dose responses, sham-feeding, and gastrin release by food, secretin and sham-feeding, and basal acid inhibition by graded doses of atropine and H-2 antagonists are being studied in duodenal ulcer and gastric ulcer and related to clinical data on age, sex, bodyweight, duration of disease, incidence of complications and response to medical and surgical therapy.